Soldering of electronic parts has primarily used the alloy Pb-63Sn (in this specification, numbers indicating an alloy composition mean mass percent), which is a eutectic solder. A eutectic solder has a relatively low melting point of 183° C. (since it is a eutectic, the solidus temperature and the liquidus temperature are the same). Since a soldering temperature of 20-50° C. above the liquidus temperature of the solder is generally considered suitable, the soldering temperature of a eutectic solder becomes approximately 200-230° C. If the soldering temperature is in this temperature range, soldering can be carried out without imparting thermal effects to electronic parts or printed circuit boards. A eutectic solder also has excellent solderability, and the eutectic solder has the advantage that the occurrence of unsoldered portions or voids in soldered portions is small.
It is possible to use a high-temperature solder having a high melting point for soldering of electronic parts. For example, a high-temperature solder is ordinarily used in soldering when assembling electronic parts such as power transistors and transformers which generate large amounts of heat when they conduct. If a eutectic solder is used, the solder melts when soldered portions reach a high temperature, or even if the solder does not melt, it increases in temperature close to the melting point of the eutectic solder, its bonding strength markedly decreases, and soldered portions sometimes peel off under small vibrations or impacts.
A high-temperature solder is also used for soldering the first time (referred to below as initial soldering) when soldering is to be again performed in the vicinity of a soldered portion (below, such soldering is referred to as step soldering). For example, step soldering takes place when an electronic part, which was assembled by soldering an electronic element to a substrate, is soldered to a main board in the form of a printed circuit board. If an electronic part is assembled by initial soldering using a eutectic solder and then the same eutectic solder is used for soldering the assembled electronic part to a printed circuit board (this soldering a second time is referred to below as second soldering), at the time of second soldering, the electronic part is exposed to a soldering temperature higher than the melting point of the eutectic solder, the initially soldered portions inside the electronic part melt, and the intended function of the electronic part can no longer be achieved. Therefore, soldering at the time of assembly of an electronic part and more typically initial soldering in step soldering is carried out using a high-temperature solder which does not melt at the second soldering temperature, i.e., at the soldering temperature of a eutectic solder.
Accordingly, it is desired that a high-temperature solder used in step soldering have a solidus temperature higher than 250° C. so as not to melt at the soldering temperature of a eutectic solder. The primary high-temperature solders which have been used up to the present time have been alloys having Pb as a main component such as Pb-5Sn (solidus temperature of 300° C. and liquidus temperature of 314° C.), Pb-10Sn (solidus temperature of 268° C. and liquidus temperature of 301° C.), Pb-5Ag (solidus temperature of 304° C. and liquidus temperature of 365° C.), and the like.
It is difficult to separately recover printed circuit board inside discarded electronic equipment, and they are often disposed of by burial in the ground. In recent years, lead pollution of underground water due to leaching by acidified rain water of Pb from soldered parts such as printed circuit boards disposed of by burial has come to be regarded as a problem. Therefore, the use of solder containing Pb is now restricted, and in its place, lead-free solder which does not contain any Pb has come to be used.
Typical lead-free solders are alloys having Sn as a main component to which one or more elements such as Ag, Cu, Sb, Zn, Bi, In, Ni, Cr, Fe, Mo, P, Ge, and Ga are added. At present, the lead-free solders which are most used are medium-temperature solders such as Sn-3Ag-0.5Cu (solidus temperature of 217° C. and liquidus temperature of 220° C.), Sn-3.5Ag (solidus temperature and liquidus temperature of 220° C.), Sn-0.75Cu (solidus temperature and liquidus temperature of 227° C.), and the like. In order to avoid thermal effects on electronic parts or printed circuit boards, a soldering temperature of 20-40° C. above the liquidus temperature is considered suitable for these medium-temperature lead-free solders, and it is recommended that soldering take place at a temperature of at most 260° C.
In order to carry out lead-free step soldering using a medium-temperature lead-free solder instead of a eutectic solder, it is necessary to carry out initial soldering using a high-temperature lead-free solder having a solidus temperature higher than 260° C. which will not melt at the temperature of second soldering using a medium-temperature solder. However, with a lead-free solder having Sn as a main component, even if it contains a large amount of high melting point metals such as Ag, Cu, Sb, Ni, Cr, Fe, and Mo, only the liquidus temperature of the solder increases, and the solidus temperature cannot be increased to at least 260° C. Thus, there are no high-temperature solders suitable for use in step soldering among lead-free solders having Sn as the main component.
Therefore, in Europe, RoHS Regulations provide the exception that solders having Pb as a main component can be used only for high-temperature solders. In Japan, although it is not a legal restriction, there is a policy that users should regulate themselves and refrain from using solder containing any Pb. Since a high-temperature lead-free solder does not exist, when step soldering is carried using a lead-free solder, the solder used in initially soldered portions remelts at the time of second soldering, and it is difficult to avoid the occurrence of cracks in soldered portions or positional deviation of electronic parts.
Soldering for the purpose of mounting electronic elements on a substrate to form electronic parts is often carried out by the reflow method using solder paste. A solder paste is formed by mixing solder powder and a flux to form a paste. A solder paste can easily be applied to prescribed locations by screen printing or the like. In the reflow soldering method, even when there are positional deviations of electronic elements disposed on a substrate, due to the surface tension of molten solder at the time of solder reflow, a self-alignment effect is achieved in which the elements are aligned on suitable locations.
In JP 2001-219294 A1 (Patent Document 1), one of the present applicants proposed a solder paste comprising a flux containing a thermosetting resin and a solder powder. After soldering is performed with this solder paste, a flux residue containing a resin remains and covers soldered portions. The bonding strength of the solder is reinforced by the adhesive strength of the thermosetting resin, so an extremely strong bonding strength can be obtained. As a result, it becomes unnecessary to carry out filling of the areas surrounding a soldered portion with resin (referred to as underfilling) in order to strengthen the bonds of soldered portions. Examples which are given of a solder powder for the solder paste are 63Sn—Pb (melting point of 183° C.), Sn-3.5Ag-0.75Cu (solidus temperature of 217° C. and liquidus temperature of 220° C.), and Sn-2.5Ag-1Bi (solidus temperature of 214° C. and liquidus temperature of 220° C.).
In JP 2005-72173 A1 (Patent Document 2), the present applicants proposed an electronic part in which at least a portion of the components thereof are soldered using a high-temperature solder comprising Bi or an alloy having Bi as a main component and having a solidus temperature of at least 260° C. and a liquidus temperature of at most 360° C., and in which the soldered portions are reinforced by flux residue containing a thermosetting adhesive.
JP 2001-170797 A1 (Patent Document 3) discloses a soldering flux containing an adhesive resin and a curing agent and a solder paste containing this flux.
In JP 2004-207494 A1 (Patent Document 4), in an electronic apparatus which is manufactured by initial soldering of an electronic element to a substrate and filling the periphery of the soldered portion with a resin (underfill) and then by second soldering to mount the electronic part which is formed on a mounting substrate (a printed wiring board), at the time of initial soldering, a solder alloy is used having a different of at most 5% between its volume as a solid and its volume as a liquid. Examples which are given of such a solder alloy are (15-80)Bi—Sn alloys and (15-80)Bi-(0.5-3)Ag—Sn alloys, but the alloys shown in the examples are only Sn-30Bi, Sn-40Bi, Sn-57Bi, and Sn-57Bi-(0.5-2.5)Ag alloys, and high Bi—Sn alloys having a Bi content of greater than 57% were not tested. A Sn-57Bi alloy, which has a composition close to the eutectic composition of a Bi—Sn based alloy, has a solidus temperature of 138° C. and a liquidus temperature of 139° C. There is no description concerning using a thermosetting resin as an underfill.
The object of Patent Document 4 is to make it possible to carry out step soldering using a lead-free solder. By using a solder alloy for which the difference between its volume as a solid and its volume as a liquid is at most 5% at the time of initial soldering and surrounding a soldered portion after initial soldering with an underfill, even if the initially soldered portion melts due to the heat at the time of second soldering, there are almost no gaps or cracks in the initially soldered portion, and the occurrence of short circuits between electrodes due to inflow of molten solder can be prevented.
However, the technology disclosed in Patent Document 4 has problems like the following.
First, in a separate step, it is necessary to completely fill the space in the periphery of the initially soldered portion with the underfill resin and cure the resin. Curing of a resin is normally carried out by heating for around 30 minutes at 130-160° C., and since this takes time, it markedly worsens productivity.
In addition, a solder alloy having a composition of Sn-(30-57)Bi which is used in the examples of Patent Document 4 has a solidus temperature and a liquidus temperature which are close to each other. Thus, when the alloy is remelted during second soldering, if a force is applied, the initially soldered portion easily breaks, and there are cases in which the initially soldered portion completely melts due to heating during operation of electronic equipment. Furthermore, Ag plating or Ag—Pd plating covering Cu electrodes of electronic parts more easily melts in this alloy than in a 63Sn—Pb eutectic alloy, and if such melting occurs, electrocorrosion of Cu electrodes sometimes occurs during conduction of electricity.
The solder alloys with a composition in the range of Sn-(30-57)Bi which are specifically disclosed in Patent Document 4 have an increase in volume at the time of melting of at least 1.0% even if they contain a fairly large amount of Ag, which has the effect of suppressing changes in volume.